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GNU bison is a free parser generator computer program written for the GNU project, and available for virtually all common operating systems. It is mostly compatible with yacc, and offers several improvements over the earlier program.However it is not fully compatible with the yacc of old. This is the source of a large number of portability problems. Some code generated by it will not even compile.
Unlike yacc, GNU Bison was written to lock its design into the GNU tools like gcc and as such needs them to compile itself. This limitation also keeps it from being as cross platform as it could as the generated code is not always compatible with ANSI C compliant tools that do not support the GNU extensions to C.
Another problem is that earlier versions of bison required that the outputted code generated had to be under the GPL. An exception allowing its output to not have to be under the GPL was added by GNU in bison version 1.24
1 Bison symbols
%left
Declaration to assign left associativity to tokens.
2 Reference
GNU project software
Computer programming tools
